The present invention relates to means and methods for installation of elongated articles from a surface vessel to a sea bed. It relates in particular to permanent installations and to the problem of preventing damage to an elongated article which is installed between a surface vessel and the sea bed. The elongated article may be a cable, an umbilical, a flexible pipe riser or the like.
A special technical problem is to provide a suitable transfer from a bottom laid static article (umbilical) to a dynamic article (umbilical) leading as a steep wave to a surface installation. One solution is described in our copending NO 961249 A (O Birkelund 3-1) by which the dynamic umbilical is secured to a bottom structure by means of a tether wire. This solution is--however--not suitable in all situations.
U.S. Pat. No. 3,216,203 relates to an arrangement for submarine conveyance of fluids through a flexible pipe line. The pipe line is assumed to have a positive buoyancy and it may be maintained at a desired depth by anchoring the pipe line with several pairs of cables to sea bed structures. This publication does, however, not give any hints to how a steep wave installation could be handled. With a steep wave installation, the elongated article will normally follow an S-curve configuration (lazy wave) between a substantially vertical outlet from a vessel and a substantially vertical installation on the sea bed. At a certain position between the vessel and the sea bed, the elongated article is suspended from buoyancy devices to create the flexible S-form.
At a lower position the elongated article is anchored to the sea bed. There is little risk of damaging the elongated article at the buoyancy device suspending positions. At the anchoring positions there are, however, transverse and in-line forces and potential uncontrolled shapes and configurations acting on the elongated article and these are difficult to cope with.